linux-kernel - Re: [PATCH 0/3] Virtual huge zero page

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20120929143737.GF26989@redhat.com>
Date:	Sat, 29 Sep 2012 16:37:37 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH 0/3] Virtual huge zero page

On Sat, Sep 29, 2012 at 07:30:06AM -0700, Andi Kleen wrote:
> On Sat, Sep 29, 2012 at 03:48:11PM +0200, Andrea Arcangeli wrote:
> > On Sat, Sep 29, 2012 at 02:37:18AM +0300, Kirill A. Shutemov wrote:
> > > Cons:
> > >  - increases TLB pressure;
> > 
> > I generally don't like using 4k tlb entries ever. This only has the
> 
> From theory I would also prefer the 2MB huge page.
> 
> But some numbers comparing between the two alternatives are definitely
> interesting.  Numbers are often better than theory.

Sure good idea, just all standard benchmarks likely aren't using zero
pages so I suggest a basic micro benchmark:

   some loop of() {
      memcmp(uninitalized_pointer, (char *)uninitialized_pointer+4G, 4G)
      barrier();
   }

> 
> > There would be a small cache benefit here... but even then some first
> > level caches are virtually indexed IIRC (always physically tagged to
> 
> Modern x86 doesn't have virtually indexed caches.

With the above memcmp, I'm quite sure the previous patch will beat the
new one by a wide margin, especially on modern x86 with more 2M TLB
entries and >= 8MB L2 caches.

But I agree we need to verify it before taking a decision, and that
the numbers are better than theory, or to rephrase it "let's check the
theory is right" :)
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/